Development, and Structure of the Vegetable Cell. 191 
composed of cell-structures, these usually exercise a perceptible 
influence on the form of the superadded layers. Both the 
organized contents and the form of the thickening layers caused 
by them afford grounds for the discrimination of the different 
layers that concur in the construction of the cell-system of a 
tissue-cell. 
For as such a cell-system is not only composed of a number 
of cells, but each of these cells again consists of many super- 
posed layers, it is often difficult to make out the essential nature 
of any single layer, especially when these lamin, as not un- 
frequently happens, are only loosely connected together, or are 
of dissimilar chemical constitution, or, again, when the mem- 
branes of various endogenous cells are of homogeneous consist- 
ence, or for other reasons are undistinguishable or inseparable 
from each other. Under such circumstances it is the rule that 
the layers of a cell-wall never contain organized bodies ; where 
such are present, the nearest external membrane is the mem- 
brane or the innermost lamina of an organized cell. 
The layers of deposit may indeed at the time of absorption 
be separated from each other by fluid materials; but they do 
not enclose organized forms. On the other hand, many endo- 
genous cells of the system of a tissue-cell contain only fluid, 
which makes their recognition as cells difficult. Most commonly, 
however, at least the secondary and the next cells in the interior 
enclose organized forms. 
When, among these organized contents of the secondary cell 
(consisting of vesicles containing secretion-material and fre- 
quently, when the cell is not engaged in the multiplication, of a 
nucleus), one of these secretion-vesicles becomes so much ex- 
tended at the expense of the others as to attain the size of the 
mother cell; the tertiary cell (the cell-nucleus) and the rest of 
the contents are enclosed between the two membranes, which 
then become approximated, and from this results the form de- 
signated by Unger the “ parietal (wandstdndiger) cell-nucleus,” 
which led Schleiden into his above-mentioned erroneous notion 
of cell-genesis. These forms are developed in fruits which are 
becoming succulent, as also in cells filled with blue, red, and 
many kinds of yellow colouring-matter. 
But commonly there is a different state of things, the small 
secretion-vesicles (chlorophyll, starch, mucus, &c.) becoming 
adherent, during their development, to the internal surface of 
the membrane of the secondary cell. At a later period, when 
this membrane begins to undergo a chemical change, and to 
thicken, these secretory matters become absorbed; the vesicles 
vanish out of sight, but the spots at which they have adhered, 
or still may adhere, do not undergo thickening. 
